I don't believe the "perfect packgoat" exists. But, there are attributes that you need to be aware of when picking your prospect. If you are a light, weekend packer you will be able to use a goat with some flaws, but the harder you intend to work your goat, the more of the following items you will have to take into consideration.

CONFORMATION

Prospects should be:
CAE and CL free-(goat viruses-CAE looks like arthritis, & CL is an abscess forming disease)
Minimum 34" at the withers-(although I have packed smaller, if you want a large load this is the minimum size.)

Minimum 200 pounds-( same comment as height)
Leg length proportionate to body (no dachshund-looking conformation)
Flat back from withers to loin-(helps carry weight evenly)
Withers and croup both same distance from the ground-not going uphill or down-(keeps pack from sliding)
Cannon bone 1/3 length of upper leg-(longer stride)
Good bone size in legs and feet
Wide across hip and shoulders-(good weight bearing surface)

Legs track reasonably straight when watched from the front
Slightly hocky-good angles to hocks when viewed from the side and back give better traction and stay sounder than posty (very straight) legs
These, of course describe the "perfect" goat, but the harder you plan to work, the more they need to have good proportion and strong bone and feet.

To de-horn or not to de-horn, that is the question. There is a lot of controversy over this. Horns work somewhat as a cooling system in goats. They are also a major liability in herd management.
The arguments pro and con are many. Whichever you decide on, all your animals should be the same. My position is that horns look really cool, but they can do a lot of damage to a person or to another goat, so in my herd we're all dis-armed.
I will sell horned kids by order, but otherwise they're all dis-budded by about 1 week.

MENTAL ATTRIBUTES

A goat can have all the great conformation in the world, but if he's not a "gung-ho" goat mentally, he's worthless as a packer. Good pack goats like to work and spend time with people. It's obvious when they're very small. In fact, my predictor for success as a packgoat is, if they follow me into the training mud puddle for their bottle, they are likely to be good packers.
Some goats are lazy. Nubians particularly have a reputation for this, but this doesn't mean that all are.

A goat from a reputable packgoat breeder will have been bred to select for "gung-ho goat" qualities. Goats should show that they are well-socialized and not exhibit fear or aggressiveness toward people. By the time they are weaned, they should not be butting or jumping up on people. It's cute in kids but not in full grown spoiled monsters. Your goat should be willing to respect you, just like any other working animal.